1. Field of the Invention
The present invention relates to a television standards conversion apparatus for converting the number of scanning lines and the field frequency of video signals into those of another television standards
2. Description of the Related Art
Conversion in television standards from a video signal of the 625/50 standard having 625 of scanning lines and a field frequency of 50 Hz to a video signal of the 525/60 standard having 525 scanning lines and a field frequency of 60 Hz is achieved by conversion in the number of scanning lines and field frequency. Such conversion in the number of scanning lines and field frequency is typically performed by digital signal processing using digital memories and so on as described in ITEJ Technical Report Vol. 14, No. 32, pp. 1-6 (June 1990).
Its configuration and operation will hereafter be described with reference to FIGS. 9 and 10 for the case where a television signal inputted to be converted is a PAL component signal of the 625/50 standard and it is to be converted into an NTSC component signal of 525/60 standard and outputted FIG. 9 is a schematic diagram showing an example of conventional scanning line interpolation processing FIG. 10 is a diagram for explaining how conventional field frequency conversion is performed by means of repetition of a signal of field unit.
A digitized component signal of PAL standard is inputted from an input terminal From the PAL signal having 625 scanning lines per frame, an NTSC signal having 525 scanning lines per frame is produced in a scanning line interpolation circuit by scanning line interpolation processing for converting spatial positions of scanning lines as shown in FIG. 9. In FIG. 9, positions of respective scanning lines correspond to spatial positions on the screen By the scanning line interpolation processing, two PAL scanning lines are weighted and combined to interpolate NTSC scanning lines as shown in FIG. 9 In the conventional example, however, a signal which is not necessary as the NTSC signal as represented by a phantom line is also interpolated in the ratio of approximately six to one. In fact, 525 scanning lines, which are required as the NTSC signal and which are represented by solid lines in FIG. 9, and 100 scanning lines which are not required as the NTSC signal, i e., a total of 625 scanning lines per frame are outputted from the scanning line interpolation circuit to a field memory. On the basis of a control signal fed from a memory control circuit, it is inhibited to write scanning lines, which are included in 625 inputted scanning lines per frame and which are not required as the NTSC signal, into the field memory, whereas only 525 scanning lines which are required as the NTSC signal are written into the field memory. That is to say, scanning line decimation is performed on 625 scanning lines to produce 525 scanning lines. At the time of writing scanning lines into the memory, scanning lines are decimated from 625 to 525 per frame, i.e., at a rate of 5/6. In reading scanning lines from the field memory, therefore, the signal can be read out repetitively by taking a field as the unit at the rate of one field every five fields as shown in FIG. 10. By reading out repetitively the signal corresponding to one field with respect to five-field input, six fields are outputted. Thus the field frequency is converted from 50 Hz of the PAL standard to 60 Hz of the NTSC standard. A component signal, which has been converted from the PAL standard to the NTSC standard by the operation heretofore described, is outputted from an output terminal.
In such a conventional scanning line number and field frequency conversion circuit in the television standards conversion apparatus, scanning line interpolation is performed and thereafter repetition of the field unit for field frequency conversion is performed. Therefore, the state in which odd fields and even fields are correctly outputted alternately is not brought about as shown in FIG. 10. Thus the field sequence relation is not maintained.
In case an odd field and an even field are outputted in a reverse order in the conventional example, therefore, the odd field and the even field are respectively outputted as an even field and an odd field by replacing a vertical synchronizing signal. When this method is used, the state in which odd fields and even fields are normally outputted and the state in which an odd field is outputted as an even field and an even field is outputted as an odd field are repeated every six fields. In the image reproduced from the converted signal of the 525/60 standard, therefore, a flicker of 10 Hz swinging finely upward and downward is caused, resulting in picture quality degradation.
For preventing such a flicker, it is considered to conduct processing of interpolating scanning lines on the readout side of the field memory. By reading out one field repetitively from the field memory every five fields, the field frequency is converted from 50 Hz to 60 Hz. This signal thus converted in field frequency is subject to scanning line interpolation processing for converting spatial positions from scanning lines of 625/50 standard to scanning lines of 525/60 standard. At this time, field attribute alteration is performed. That is to say, if the field sequence relation of the output signal holds true, scanning line interpolation processing from an odd field of 625/50 standard to an odd field of 525/60 standard or scanning line interpolation processing from an even field of 625/50 standard to an even field of 525/60 standard is performed. If the field sequence relation of the output signal is reversed, scanning line interpolation processing from an odd field of 625/50 standard to an even field of 525/60 standard or scanning line interpolation processing from an even field of 625/50 standard to an odd field of 525/60 standard is performed. By doing so, the field sequence relation of the output signal is always maintained and an interpolated scanning line is outputted on the correct position on the screen. Therefore, flickers swinging finely upward and downward are not caused.
In case scanning line interpolation processing is performed on the readout side of the field memory as described above, however, the signal of one field is repetitively read out from the field memory every five fields to convert the field frequency from 50 Hz to 60 Hz. For thus reading the signal of one field repetitively, the number of scanning lines per field must be limited to 5/6. That is to say, readout must be performed while decimating one scanning line every six scanning lines. As a result, precision of scanning line interpolation in the position corresponding to the scanning line decimated on the converted screen is degraded.